The present disclosure relates generally to halftoning and, in particular, to multibit digital halftoning using lookup tables.
Halftoning refers to a process or technique in image reproduction that utilizes varying sizes of dots (and/or varying degrees of spatial separation among these dots) to simulate a continuous tone image using a single color of ink (i.e., without the use of a wide or infinite range of continuous tones) by the arrangement of halftone dots such that they appear blended to the human eye when viewed at an appropriate distance.
Digital halftoning techniques have been developed for implementing halftone images. Digital halftoning generates a bitmap within which each monochrome picture element or pixel may be on or off, or set to ink or no ink. The bitmap corresponds bit-for-bit with an image (e.g., displayed on a computer screen or printed on paper). A digital halftone cell (whereby a cell relates to a corresponding area (size and location) of the continuous-tone input image) contains groups of monochrome pixels within the same-sized cell area.
Currently, a widely used class of digital halftoning techniques utilize look up tables (LUTs) that produce quality halftone images, but typically require a significant amount of memory. For example, for multibit digital halftoning, a three-dimensional (3D) look up table is utilized that accommodates two-dimensional (2D) spatial data for the image and one-dimensional (1D) gray levels. The spatial 2D data potentially require entries in the order of 100,000 s, and with the number of input gray levels in the range of 8-10 bits per table entry, this results in a table with potentially millions or tens of millions of entries. In another solution, a two-dimensional (2D) look up table and a one-dimensional (1D) look up table are used to reduce the storage requirements associated with 3D look up table implementations; however, this solution lacks the flexibility of a fully three-dimensional (3D) lookup table, as the number of possible 3D look up tables far exceeds the number of possible 2D or 1D lookup tables.
What is needed, therefore, is a way to perform digital halftoning for images that offers flexibility of implementation while minimizing the amount of storage required for the implementation.